1. Field of the Invention
The invention relates to a memory element, and more particularly to a phase-change memory element.
2. Description of the Related Art
Phase-change memory technology requires high reliability, high speeds, low current, and low operating voltage, in order to function as a viable alternative to current memory technologies such as flash and DRAM. A phase-change memory cell must therefore provide low programming current, voltage operation, a smaller cell size, a fast phase transformation speed, and a low cost. These requirements are difficult to meet given the current state of the art.
The phase-change memory structures are predominantly self-heating, i.e. current flows through the phase-change material to produce the required heat. The top electrical contact of the phase-change memory is generally wider than the bottom electrical contact area. The structure may be a pore filled with phase-change material, or a block of phase-change material with a sublithographic bottom contact, and lithographically defined top electrode. The wider contact defines the maximum current required to write a bit to the cell. Variations in the bottom or top contact width due to lithography, etching or other processing stages result in variations of the required programming current.
Ideally, the phase-change memory will have a fixed cross-section area, such as a pillar or bridge. However these structures tend to require high voltages as the cross-section area and/or the length is reduced. An issue limiting further advancement is that the electrodes connecting to the structure, act as heat sinks.
Macronix (U.S. Pub. 20060284157 and U.S. Pub. 20060284158) disclosed a basic phase-change bridge structure. However, the electrode contacts are the only contacts to the bridge and heat significantly flows out through these contacts, thus making programming very inefficient.
U.S. Pat. No. 7,119,353 discloses a phase change memory element, including a substrate, a CMOS formed on the substrate, a dielectric layer, a metal plug, and a phase change memory cell. Particularly, the phase change memory cell includes a phase change material layer, and a pair of electrodes. The CMOS electrically connects to the electrode of the phase change memory cell via the metal plug. Accordingly, the active area (phase change area) can be determined by the thickness of the phase change material layer. The contact surface between the phase change material layer and the metal plug, however, is limited by the diameter of the metal plug, thereby making it unable to increase heating efficiency and reducing the programming current of the phase change memory cells.
Therefore, it is desirable to devise a phase-change memory cell structure that improves upon the aforementioned problems.